1. Field of the Invention
This invention is for energy dissipators such as may be linked with water turbine relief and bypass valves or irrigation or water works discharge regulating valves.
2. Description of Related Art
When transferring water from high level canals to low level canals through a penstock, it is necessary to control the water at the penstock outlet by means of a discharge regulating valve.
The velocity of the water emerging from the valve is Cv..sqroot.2gH where
Cv is the coefficient of velocity PA1 g is the acceleration due to gravity; and PA1 H is the height from head water level to the valve outlet minus the loss of head arising from friction in the penstock.
If the low level canal is replaced by a lake of some width, length and depth, then the kinetic energy of the effluent can be destroyed either by angling the jet to 45.degree. from the horizontal into the air or by discharging under the water in a horizontal direction. A common method is to use a hollow valve with a conical base disposed with its apex directed upwards towards the incoming flow of water so that it spreads the water over a large area, and rapidly reduces the thickness of the annular jet, thereby reducing the concentration of kinetic energy and the propensity for destruction.
When however a canal of small lateral dimensions is used to conduct the effluent away it is not possible to use any of the means described above, since the water would jet over the canal or if discharged into the canal would rapidly erode the bed and walls, especially at very high heads.
The problem is most acute when the discharge regulating valve is combined with a turbine relief valve which discharges into the turbine draft tube. As a relief valve it may only operate a few times a year for 60 seconds at a time and hence it is unlikely to do any permanent damage to the draft tube and the tail race structures. However, when the same valve is used continuously as a turbine by-pass valve considerable damage can be done in a matter of days.
The remedy is to fit an energy dissipator after the turbine relief and by-pass valve and so reduce the velocity to an acceptable level where it will not do any damage to the downstream structures.
Energy dissipation can be effected by staging the destruction of energy by causing the water to contract and expand suddenly. This method is feasible provided that the number of stages is such that cavitation does not take place at the points of contraction. It therefore calls for a relatively large dissipation chamber. It also creates a back pressure which in turn requires a larger discharge regulating valve. However a serious draw-back is that the energy dissipation is proportional to discharge squared, hence at low discharges it is less effective than at high discharges. Furthermore, the pressure drop across the discharge regulating valve is no longer constant.
A way around this problem is to employ a multi-stage valve, but this again is expensive and has to be carefully designed to ensure that cavitation cannot occur. The ultimate stage in a high head installation can be several times larger than the first stage valve.
When either of the foregoing expedients are employed, the mismatch with turbine characteristics may render them unsuitable for relief valve duty as the mismatch may be conducive to water hammer.
The ideal discharge regulating valve is one which has a linear discharge-stroke characteristic, discharges freely into a chamber which is well ventilated to atmosphere and where the process of energy dissipation is not influenced by changes in flowrate.
Discharge regulating valves/turbine relief valves are conventionally either needle/mushroom valves or variants of the hollow conical-based valve. The former type produces a concentrated cylindrical jet which has to be dispersed by making it strike symmetrically onto the apex of a cone. The hollow conical-based type of valve automatically spreads the jet into a conical fan shape and by virtue of conservation of mass and density the jet becomes a relatively thin annulus which reduces in thickness according to the inverse square law.